1. Field of the Invention
The present invention is an apparatus, in the following referred to as high frequency matching network, to induce a high frequency electric alternating field into a low pressure gas discharge, in the following referred to as plasma, while simultaneously matching the load impedance to the interior resistance of a commercial high frequency generator.
2. Description of the Prior Art
Currently high frequency excited plasmas have applications in many technological areas, especially in the treatment of solid state surfaces. Thus high frequency plasmas are used for example for deposition of thin films or cleaning and etching of surfaces. When using high frequency plasmas the use of an impedance matching network is necessary as to tune the load impedance to the interior resistance of the utilized high frequency generator. Only if the load impedance is equal to the interior resistance of the generator the optimal transmittance of power from the generator to the load is possible. Generally, there is an impedance matching network between load and generator, which provides the matching of the two impedances.
According to the current state of technology matching networks for plasma excitation permit only the inductive matching of the load impedance to the interior resistance of the generator, to minimize the reflected power and hence optimize the power induced into the plasma. An alternating electrical field is coupled into the plasma by an electrode in contact with the plasma or by irradiation of the field through a dielectric medium. For the excitation efficiency of the plasma it is of special significance whether the capacitive or inductive excitation prevails. A capacitive excitation of the plasma is characterized in that the electromagnetic wave is irradiated into the plasma through its boundary and is exposed to exponential damping when expanding towards the center of the plasma. An inductive excitation is characterized in that there is a coupling of the alternating electric field with an induced alternating magnetic field inside the plasma. Generally the excitation is a mixed excitation with a capacitive and inductive fraction. At a fixed supplied high frequency power the capacitive part of the plasma coupling is determined by the voltage amplitude and the inductive part of the plasma coupling by the current amplitude is determined by the current amplitude of the alternating electric field applied to the electrode.
Rayner, Cheetham and French described in J.Vac.Sci. Technol. A 14(4) (July/August 1996), pages 2048-2055 a high frequency matching network, which consists out of two electrical circuits, which are coupled by the inductive flux of high frequency air core inductive coils.